Thermoplastic resins, as opposed to cross-linked thermosetting resins, possess a tendency to cloud and undergo discoloration under certain conditions. This effect is caused in part by polymeric decomposition when exposed to elevated temperatures, or contact with moisture or oxygen. Among the many thermoplastic resins, molded and extruded polycarbonate thermoplastics are in great demand due to their high resiliency, resistance to attack by chemical solvents, high heat stability, non-toxicity and clear, colorless film forming properties, many of which properties are lacking in thermosetting resins such as those disclosed in U.S. Pat. No. 3,917,555.
High clarity and water-white color are particularly important characteristics in polycarbonate products which demand an exceptionally high degree of colorless transparency. Further, polycarbonates of high molecular weight are generally desired for hard, resilient, transparent and colorless products such as bullet-proof safety glass, street light lenses, bottles, automotive light shields, kitchen appliances, safety glasses, packaging films and others. Clear, colorless polycarbonate sheets are particularly desirable in applications where transmission of light is required, as in the lenses of automotive headlights. Prior polycarbonate lenses have tended to yellow with time. Accordingly, it has been the aim of research to develop a polycarbonate with eliminates or minimizes this disadvantage. In U.S. Pat. No. 4,431,673 there are disclosed certain cosmetic formulations containing certain guerbet alcohol derived esters. However, such cosmetic uses do not require colorless transparency in the final product. Exceptional uniformity is also demanded for precision molded articles of high molecular weight polycarbonate thermoplastics used in laser read compact recording discs, microwave cookware and containers for comestible or medical products.
The thermoplastic moldable polycarbonates require special care in processing since many difficulties which are absent from thermosetting resins and which are unique to these resins are encountered. Notable among the difficulties in molding is the tendency of polycarbonates to degradation (i.e. discoloration and generation of carbon dioxide bubbles) in the presence of small amounts of water or methyl alcohol. It is also known that atmospheric oxygen is troublesome since it not only causes discoloration but also induces cross-linking, a condition to be avoided where resiliency and absence of color are required (Encyclopedia of Chemical Technology, Kirk-Othmer, 2nd Edtion, Vol. 6 page 108). Thus, the kneading steps employed in U.S. Pat. No. 3,917,555 for thermosetting resins, and other operations which may introduce oxygen or moisture, are strictly prohibited. It has also been suggested that the presence of esters may be detrimental since certain species swell the polymer (Encyclopedia, Ibid), tend to migrate to the polymer surface and cause stress points which lead to cracking in molded sheets during storage.
Localized overheating of thermoplastic resins may occur during the extrusion melting process wherein frictional forces generated within the extruder or other malaxing device are employed to melt the thermoplastic resin. The presence of a lubricating component such as mineral oil is previously known in the art to assist in polymer melt lubrication and reduction in thermoplastic polymer degradation.
In addition to the foregoing effects molten polycarbonate resins in particular are known to be relatively viscous. Such viscosity is affected by molecular weight and temperature. In the preparation of molded objects the reduced melt viscosity of a thermoplastic resin allows reduced molding cycle times thereby generating greater efficiency. Within design restraints it may be possible to attain decreased melt viscosity only by use of increased temperatures or reduced polymer molecular weight. The disadvantage in using elevated molding temperatures in increasing polymer degradation have been previously disclosed. Lower molecular weight polymers generally possess inferior strength properties, in particular, reduced impact and tensile properties. Accordingly there exist molding constraints which may prevent improved molding efficiency by either increased molding temperatures or reduced polymer molecular weight.
An improved internal lubricant may, however, also serve to reduce the melt viscosity of the molten resin in which it is incorporated allowing the successful molding of higher molecular weight resins compared to resins lacking such lubricant or alternatively allowing reduced injection molding times for resins of the same molecular weight. Ideally an improved internal lubricant for thermoplastic resins would possess desirable properties in both decreasing polymer degradation and reducing polymer melt viscosity.
Mold release additives perform the desirable function of permitting easy release of the molded object from the surface of the mold cavity after resolidification of the thermoplastic resin. Such additives are desirably incorporated into the polymeric resin itself, instead of being physically applied to the mold surface by the operator. Such mold release additives are referred to in the art as internal mold release additives. Sometimes, but not always, a composition may act both as a lubricant for the polymer and as a mold release additive. Thus it cannot be determined a priori from the fact of a composition's known lubricating ability that such composition also will possess good mold release properties. Not only must the composition be successful in reducing the force needed to remove the molded object from the mold, but the material must be compatible with thermoplastics and not cause polymer degradation especially at elevated temperatures. Also the composition should not cause deposition or plate out to occur on the mold surfaces (necessitating frequent mold cleaning), and should not contaminate the surface of the resulting molded object. The latter property allows coextrusion to other thermoplastic resin layers or subsequent operations such as painting, laminating, etc. to be performed without first cleaning the surface of the molded object.
A further desirable feature of the foregoing additives is that the same be a liquid which allows ease of handling and incorporation into the thermoplastic resin.
Also, it would be desirable if there were provided a composition for use with thermoplastic molding resins which combines good lubricating properties and good mold release properties in one composition, thereby reducing the number of additives that need to be incorporated into the resin.
It would additionally be desirable to provide a composition which is an efficient lubricant or mold release additive thereby reducing the amount needed to be incorporated into the polymer in order to achieve the desired result. Reduction of the amount of additive employed not only is more economical but additionally results in less overall deterioration of polymer physical properties.
Finally it would be desirable to provide a lubricant which may be surface coated onto particulated thermoplastic resins to provide good solid particle lubrication (thereby achieving improved solid flow and handling properties and improved extruder feed and mixing) which properties are enhanced due to concentration of the additive as an evenly dispersed film on the surfaces of the particulated thermoplastic resin; subsequent effective incorporation into the polymeric melt for good melt lubrication; and desired mold release performance.
It is an object of this invention to overcome the above disadvantages and to provide at least some of the desirable lubricant features discussed above.